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Python Exception Hierarchy

Exceptions are organized in a family tree, with general errors as parents and specific ones as children. Knowing the tree helps you catch the right problem.

In this page:

  1. The BaseException Class
  2. The Exception Class
  3. The ArithmeticError Family
  4. The LookupError Family
  5. Order of Catching
Syntax
python 
try:
    # code
except SpecificError:
    # handle specific first
except Exception:
    # then more general

The BaseException Class

BaseException sits at the very top of Python's exception tree -- every exception, including system-level ones like SystemExit and KeyboardInterrupt, ultimately inherits from it, which is exactly why catching 'except Exception:' deliberately does NOT catch those two (they skip past Exception in the hierarchy).

Example: The BaseException Class

python 
print(issubclass(SystemExit, BaseException))
print(issubclass(SystemExit, Exception))

The Exception Class

Exception is the parent of nearly every normal error your code will raise or catch, and it's the class the documentation recommends your own custom exceptions inherit from -- catching except Exception: is broad but still excludes those system-exit signals.

Example: The Exception Class

python 
try:
    int("abc")
except Exception as e:  # parent of nearly every normal error
    print("Caught by Exception:", type(e).__name__)

The ArithmeticError Family

ArithmeticError groups math-related failures under one umbrella -- ZeroDivisionError (dividing by zero) and OverflowError (a result too large to represent) both inherit from it, so catching ArithmeticError handles either without listing them separately.

Example: The ArithmeticError Family

python 
try:
    1 / 0
except ArithmeticError:  # ZeroDivisionError inherits from ArithmeticError
    print("Caught a ZeroDivisionError via ArithmeticError")

The LookupError Family

LookupError groups the two most common couldnt find that' errors -- IndexError (list index out of range) and KeyError (dictionary key missing) -- letting you write one except clause that handles either kind of lookup failure.

Example: The LookupError Family

python 
try:
    [1, 2][5]
except LookupError:  # IndexError inherits from LookupError
    print("Caught an IndexError via LookupError")

Order of Catching

Because except clauses are checked top to bottom and the first match wins, you must list more specific exception types before their more general parent classes -- an except Exception: block placed first would silently swallow every specific handler written after it.

Example: Order of Catching

python 
try:
    1 / 0
except ZeroDivisionError:  # more specific handler, checked first
    print("Specific handler runs first")
except Exception:
    print("This would swallow everything if placed first")
Related Topics

Python Exception Handling · Python Custom Exceptions · Python finally & else · Python try-except

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Common Mistakes
  1. Ordering except blocks from general to specific, as in except Exception before except ValueError, so the specific handler never runs.
  2. Catching BaseException, which also catches SystemExit and KeyboardInterrupt.
  3. Confusing KeyError and IndexError, when both are LookupError subclasses but KeyError is for dictionaries and IndexError for sequences.
Chapter Summary
  • Exceptions handle errors with try-except, plus finally and else.
  • Custom exceptions define your own error types.
  • The exception hierarchy shows how exceptions relate.
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